Lazy initialization


In computer programming, lazy initialization is the tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed. It is a kind of lazy evaluation that refers specifically to the instantiation of objects or other resources.
This is typically accomplished by augmenting an accessor method to check whether a private member, acting as a cache, has already been initialized. If it has, it is returned straight away. If not, a new instance is created, placed into the member variable, and returned to the caller just-in-time for its first use.
If objects have properties that are rarely used, this can improve startup speed. Mean average program performance may be slightly worse in terms of memory and execution cycles, but the impact of object instantiation is spread in time rather than concentrated in the startup phase of a system, and thus median response times can be greatly improved.
In multithreaded code, access to lazy-initialized objects/state must be synchronized to guard against race conditions.

The "lazy factory"

In a software design pattern view, lazy initialization is often used together with a factory method pattern. This combines three ideas:

ActionScript 3

The following is an example of a class with lazy initialization implemented in ActionScript:

package examples.lazyinstantiation

Basic Usage:

package

C

In C, lazy evaluation would normally be implemented inside a single function, or a single source file, using static variables.
In a function:

  1. include
  2. include
  3. include
  4. include
struct fruit ;
struct fruit *get_fruit
/* Example code */
int main

Using a single source file instead allows the state to be shared between multiple functions, while still hiding it from non-related functions.
fruit.h:

  1. ifndef _FRUIT_INCLUDED_
  2. define _FRUIT_INCLUDED_
struct fruit ;
struct fruit *get_fruit;
void print_fruit_list;
  1. endif /* _FRUIT_INCLUDED_ */

fruit.c:

  1. include
  2. include
  3. include
  4. include
  5. include "fruit.h"
static struct fruit *fruit_list;
static int seq;
struct fruit *get_fruit
void print_fruit_list

main.c:

  1. include
  2. include
  3. include "fruit.h"
int main

C#

In.NET Framework 4.0 Microsoft has included a Lazy class that can be used to do lazy loading.
Below is some dummy code that does lazy loading of Class Fruit

var lazyFruit = new Lazy;
Fruit fruit = lazyFruit.Value;

Here is a dummy example in C#.
The Fruit class itself doesn't do anything here, The class variable _typesDictionary is a Dictionary/Map used to store Fruit instances by typeName.

using System;
using System.Collections;
using System.Collections.Generic;
public class Fruit
class Program

A fairly straightforward 'fill-in-the-blanks' example of a Lazy Initialization design pattern, except that this uses an enumeration for the type

namespace DesignPatterns.LazyInitialization

C++

Here is an example in C++.

  1. include
  2. include
  3. include
class Fruit ;
// static
std::map Fruit::types;
// Lazy Factory method, gets the |Fruit| instance associated with a certain
// |type|. Creates new ones as needed.
Fruit* Fruit::GetFruit
// For example purposes to see pattern in action.
void Fruit::PrintCurrentTypes
int main
// OUTPUT:
//
// Number of instances made = 1
// Banana
//
// Number of instances made = 2
// Apple
// Banana
//
// Number of instances made = 2
// Apple
// Banana
//

Crystal


class Fruit
private getter type : String
@@types = of String => Fruit
def initialize
end
def self.get_fruit_by_type
@@types ||= Fruit.new
end
def self.show_all
puts "Number of instances made: #"
@@types.each do |type, fruit|
puts "#"
end
puts
end
def self.size
@@types.size
end
end
Fruit.get_fruit_by_type
Fruit.show_all
Fruit.get_fruit_by_type
Fruit.show_all
Fruit.get_fruit_by_type
Fruit.show_all

Output:

Number of instances made: 1
Banana
Number of instances made: 2
Banana
Apple
Number of instances made: 2
Banana
Apple

Haxe

Here is an example in Haxe
class Fruit
Usage
class Test

Java

Here is an example in Java.

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
public class Program
enum FruitType
class Fruit

Output

Number of instances made = 1
Banana
Number of instances made = 2
Banana
Apple
Number of instances made = 2
Banana
Apple

JavaScript

Here is an example in JavaScript.

var Fruit = ;
Fruit.getFruit;
Fruit.printCurrentTypes;
Fruit.getFruit;
Fruit.printCurrentTypes;
Fruit.getFruit;
Fruit.printCurrentTypes;

Output

Number of instances made: 1
Apple
Number of instances made: 2
Apple
Banana
Number of instances made: 2
Apple
Banana

PHP

Here is an example of lazy initialization in PHP 7.4:

header;
class Fruit
Fruit::getFruit;
Fruit::printCurrentTypes;
Fruit::getFruit;
Fruit::printCurrentTypes;
Fruit::getFruit;
Fruit::printCurrentTypes;
/*
OUTPUT:
Number of instances made: 1
Apple
Number of instances made: 2
Apple
Banana
Number of instances made: 2
Apple
Banana
  • /

Python

Here is an example in Python.

class Fruit:
def __init__ -> None:
self.item = item

class Fruits:
def __init__ -> None:
self.items =

def get_fruit -> Fruit:
if item not in self.items:
self.items = Fruit

return self.items
if __name__ '__main__':
fruits = Fruits
print
print

Ruby

Here is an example in Ruby, of lazily initializing an authentication token from a remote service like Google. The way that @auth_token is cached is also an example of memoization.

require 'net/http'
class Blogger
def auth_token
@auth_token ||=
&&
get_token_from_http_response
end
# get_token_from_http_response, uri and params are defined later in the class
end
b = Blogger.new
b.instance_variable_get # returns nil
b.auth_token # returns token
b.instance_variable_get # returns token

Scala

has built-in support for lazy variable initiation.

scala> val x =
Hello
x: Int = 99
scala> lazy val y =
y: Int =
scala> y
Hello!!
res2: Int = 31
scala> y
res3: Int = 31

Smalltalk

Here is an example in Smalltalk, of a typical accessor method to return the value of a variable using lazy initialization.

height
^height ifNil: .

The 'non-lazy' alternative is to use an initialization method that is run when the object is created and then use a simpler accessor method to fetch the value.

initialize
height := 2.0
height
^height

Note that lazy initialization can also be used in non-object-oriented languages.

Theoretical computer science

In the field of theoretical computer science, lazy initialization is a technique to design data structures that can work with memory that does not need to be initialized. Specifically, assume that we have access to a table T of n uninitialized memory cells, and want to assign m cells of this array, e.g., we want to assign T := vi for pairs,..., with all ki being different. The lazy initialization technique allows us to do this in just O operations, rather than spending O operations to first initialize all array cells. The technique is simply to allocate a table V storing the pairs in some arbitrary order, and to write for each i in the cell T the position in V where key ki is stored, leaving the other cells of T uninitialized. This can be used to handle queries in the following fashion: when we look up cell T for some k, we can check if k is in the range : if it is not, then T is uninitialized. Otherwise, we check V is uninitialized. Otherwise, we know that T is indeed one of the initialized cells, and the corresponding value is the second component of the pair.